1. Field of the Invention
This invention relates to chain saws and in particular it relates to an improved drive sprocket for propelling a saw chain around a guide bar on a chain saw.
2. Background of the Invention
Motorized chain saws utilize a drive sprocket coupled to an output shaft of the drive motor to propel an endless saw chain around an elongate guide bar. Drive sprockets are basically of two different types and the present invention is directed to that type referred to as a rim sprocket.
Saw chain as contemplated herein is an endless sequence of pivotably connected links including alternating center drive links and paired side links. Certain of the side links have outwardly extended cutting portions and are referred to as cutting links. All other side links are referred to as tie straps.
The conventional rim type drive sprocket is circular in side view and has formed pockets open to the periphery and extending radially inward for receiving drive tangs that depend from the center drive links of the saw chain. The outer peripheral edges of the sprocket on either side of the pockets provide spaced rims which support the side links of the saw chain.
The lower section of the side links, whether they are cutter links or tie straps, have the same configured profile. The side links have leading and trailing flat bottom support edges (having reference to the direction of travel) separated by a center notch. These support edges are sometimes referred to as leading and trailing footprints.
The saw chain is entrained, under tension, around the peripheral edge of the elongate guide bar and drive sprocket, and is propelled by the driven drive sprocket engaging the depending tangs of the drive links. The guide bar edge has a center groove in which the depending tangs of the drive links travel to guide the chain as it travels around the bar. The bottom support edges of the side links ride on the side rails on either side of the center groove.
An inconsistency that has been recognized for years is that the saw chain is made up of short rigid links that take the shape of a polygon as the chain travels around the bar ends, whereas the rim sprocket provides a circular support edge for the chain. As the links are directed onto the sprocket, the lower front corner (the toe portion of the leading footprint) of each side link initially engages the circular support surface of the rim. As rotation progresses, the link rocks backward and in the process slides the point of engagement across the bottom edge of the leading footprint until the inner corners of both footprints (heel and toe portions, respectively) are engaged and supported on the sprocket rim.
This procedure causes several problems. As the side links rock, the connecting rivets rise and fall and the tension in the chain is varied. Because the chain is travelling at extremely high speeds (reaching upwards of 70 miles per hour), the initial engagement of the side links with the sprocket becomes a rapidly repetitive hammering that accelerates wearing of both the side links and sprocket.
Previous attempts to conform the sprocket shape to the polygon configuration in order to alleviate this hammering effect have included providing a polygon configured rim sprocket that was matched to the polygonal shape of the side links when forced into a curve. A flat side of the sprocket would rotate under each side link and provide a rigid flat surface for supporting the footprints. However, very little improvement was experienced with this arrangement. The failure of these prior attempts was believed to be due to the failure to recognize that the sprocket generates a circular path regardless of its peripheral configuration whereas the rigid links are converting from a straight line of travel to a curved line of travel. The same hammering and rocking of the link takes place in that the side link first engages and is then forcefully pivoted into position around the sprocket.